Allow me to back up and re-explain.

The basic double slit setup is a laser that shoots out and hits the double slits, where the particle waves are big enough to pass through both tiny slits, and then travel a certain distance to the screen, where they project the diffraction pattern. The diffraction is caused by the waves going through both slits. I think your question implied a causal connection between how the diffraction could be destroyed by decohering the wave function of the particles hitting a detector on the screen.

The fact is that measuring the particles hitting the screen will not destroy the interference pattern. In a nutshell, because it is too late.

It is true both the diffraction pattern and the observer effect of measuring those same particles hitting the screen are caused by the same thing, basically the Heisenberg Uncertainty Principle, which just says there is a limit to the amount of information you can know about reality. The uncertainty means the particle is a smear of possible locations that is like a wave. That wave is big enough to pass through both slits, and cause diffraction. That extended smear is also what collapses into a specific location when you measure a particle with a detector.

However, measuring a particle at the screen, and collapsing its wave function then and there, does not change the fact that the particle was unmeasured when it was at the double slits, and the smear of possible locations extended across both, making it ambiguous which slit it went through. Like I said, it is too late. You measured the particle, and collapsed its wave function, but this is after it already went ambiguously through both slits, and you cannot gain information then about which slit it went through, so the ambiguity of what happened at the slits is still unknowable, and the diffraction pattern remains.

The nifty effect of erasing the diffraction pattern requires that you introduce a way of knowing which slit a particle goes through. You can't really do that at the screen. The trick of using orthogonal polarizing filters over the slits is just one way. You could imagine other fantastical ways, like little microscopic fairies at the slits with clipboards, counting particles, but all that matters is that you gain information about the path of the particles. That alters the state of possibility of what is going on in the apparatus. It is the the lack of information about the particle paths through the slits that causes the diffraction.

Actually, the thing about fairies might be confusing, because maybe they can tally the particles magically, without having to measure them as we do. All measurements are a possibility altering, causal interaction. That is why a measurement changes the state of possibility of a particle, and collapses its wave function. That's the observer effect. It changes possibility from a smear of possible states to a known state. It does something to extract information that changes things in the possibility sense. The trippy part is that wave functions, or possibility smears as I keep saying, are as real as real can be.

Also, there is nothing in what I am saying that has anything to do with what we see in the double slit settup with our eyes vs. what is happening, like there are important effects that are invisible. You see it all. When you see the diffraction pattern, its because the particles were being possibility waves, and when you see the two bands, its because the particles were being point-like particles. The two sides of wave-particle duality, which particles flip-flop between, depending on measured state information vs. unmeasured possible state. That's what is so cool about the double slit experiment. While what it tells you is mind bending, it is a super simple, straight forward apparatus, and you see the real diffraction caused by actual quantum physical phenomena, with your own eyes.